LED lighting has become popular in the industry due to the many advantages that this technology provides. For example, LEDs typically have a longer lifespan, pose fewer hazards, and provide increased visual appeal when compared to other lighting technologies, such as compact fluorescent lamp (CFL) or incandescent lighting technologies.
Often times, LED driver circuits are configured with a dimming functionality that provides variable light output. One known technique that has been used for analog LED dimming is phase-angle dimming, which may be implemented using either leading-edge or trailing-edge phase-control. A TRIAC circuit is commonly used to perform this type of phase-angle dimming; it operates by delaying the beginning of each half-cycle of alternating current (ac) power, or trimming the end of each half-cycle of ac power. By delaying the beginning of each half-cycle, or trimming the end of each half-cycle, the amount of power delivered to the load (e.g., a string of LEDs) is reduced, thereby producing a dimming effect in the light output of the LEDs. In many applications, the delay in the beginning of each half-cycle or trimming of the end of each half-cycle is not noticeable because the resulting variations in the phase-controlled line voltage and power delivered to the LEDs occur more quickly than can be perceived by the human eye.
Flickering in LEDs may result when they are driven by LED driver circuits having regulated power supplies that provide regulated current and voltage to the LEDs from ac power lines. Unless the regulated power supplies that drive the LEDs are designed to recognize and respond to voltage signals from TRIAC dimming circuitry in a desirable way, the TRIAC dimming circuitry can produce non-ideal results, such as limited dimming range, flickering, blinking, and/or color shifting in the LEDs.
Part of the difficulty in using TRIAC dimming circuitry with LEDs is due to the characteristic of the TRIAC, which is a semiconductor component that acts as a controlled ac switch. The TRIAC behaves as an open switch to an ac voltage until it receives a trigger signal at a control terminal, causing the switch to close. The switch remains closed as long as the current through the switch is above a value referred to as the “holding current.” Most incandescent lamps draw more than the minimum holding current from the ac power source to enable reliable and consistent operation of a TRIAC. However, the relatively low currents drawn by LEDs from efficient power supplies may not meet the minimum holding currents required to keep the TRIAC conducting for reliable operation. As a result, the TRIAC may trigger inconsistently. In addition, due to the inrush current charging the input capacitance and because of the relatively large impedance that the LEDs present to the input line, a significant ringing may occur whenever the TRIAC turns on. This ringing may cause even more undesirable behavior as the TRIAC current may fall to zero and turn off the LED load, resulting in a flickering effect.
To combat these problems, conventional LED drivers typically rely on sinking additional current drawn by a dummy load or bleeder circuit to supplement the current drawn by the LEDs in order to draw a sufficient amount of current to keep the TRIAC conducting reliably after it is triggered. One drawback of conventional bleeder circuits is that certain fault conditions in the LED driver may be interpreted by the bleeder circuits as the deficiency of the load current, thus causing the bleeder circuit to draw unusually high currents. This may result in a permanent thermal failure.
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